1. Field of the Invention
The present invention generally relates to flow rate measurement of gases and more particularly to an output control of a flow sensor used in gas meters including a propane gas utility meter for flow rate measurement of a gas. Further, the output control of flow sensor of the present invention is applicable also to a flow sensor used in automobiles for air intake control. Particularly, the present invention relates to an output control of a composite flow meter that uses a fluidic sensor and a flow sensor for flow rate measurement of a gas.
2. Discussing of the Background
Flow-rate measurement of a gas is generally carried out by using a fluidic sensor. A fluidic sensor provides a reliable flow rate measurement as long as the gas flow rate is sufficiently large. When the gas flow rate is reduced, on the other hand, the sensitivity of the fluidic sensor decreases rapidly and the result of the flow rate measurement becomes no longer reliable.
Thus, in order to augment the foregoing decrease of sensitivity of fluidic sensors, it has been practiced to use a flow sensor of a different type in a flow meter, in addition to a fluidic sensor, so that a satisfactory sensitivity of flow rate measurement is guaranteed even in such a case in which the gas flow rate is very small. By using a heat-sensitive flow sensor for this purpose, for example, it is possible to construct a composite flow meter that has a high sensitivity of gas flow rate measurement from a very small flow rate value to a very large flow rate value.
In the case of a propane gas utility meter, there exists a prescribed standard or specification that imposes requirements that:
a) the gas utility meter should not produce output when the gas flow rate is 0 liter/hour; PA0 b) the gas utility meter should be able to detect a gas flow when the gas flow rate has exceeded 3 liter/hour; PA0 c) the gas utility meter should be able to detect the gas flow rate within an error of .+-.3% when the gas flow rate is in the range of 125-250 liter/hour; and PA0 d) the gas utility meter should be able to detect the gas flow rate within an error of .+-.1.5% when the gas flow rate is in the range of 250-2500 liter/hour.
As the flow meter should not produce output when the flow rate is zero (0 liter/hour), the flow meter is provided with a dead zone in correspondence to the zero flow rate value, wherein the dead zone may have a width defined by an upper limit of 1.5 liter/hour in view of the requirement that the gas utility meter should be able to detect a gas leakage or gas flow when the gas flow rate has exceeded 3 liter/hour, which is twice as large as the foregoing width of the dead zone. Thus, a gas utility meter is designed to produce a zero output indicative of the flow rate of 0 liter/hour when the actual gas flow rate is smaller than 1.5 liter/hour. Further, there exists a standard or specification imposing a requirement that the aging of the gas utility meter output should be within .+-.2% in ten years.
In view of the foregoing various requirements prescribed in the form of standard, conventional gas utility meters employ various corrections for correcting the output of a flow sensor used therein in conformity with the standards.
For example, the Japanese Laid-Open Patent Publication 3-264821 describes a composite gas flow meter that uses a fluidic sensor for flow rate detection in the flow rate range of 125-2500 liter/hour in combination with a flow sensor that measures the gas flow rate in the range of 0-150 liter/hour, wherein the fluidic sensor is used for calibrating the output of the gas flow sensor in the flow rate range of 125-150 liter/hour in which both the output of the fluidic sensor and the output of the flow sensor are available.
Further, there is proposed, according to the Japanese Laid-Open Patent Publication 4-208818, a method of correcting the zero-point of a gas flow sensor indiscriminately by regarding the gas flow sensor output indicating a flow rate value smaller than a predetermined threshold flow rate value of the dead zone, as the zero flow rate. In other words, the foregoing conventional method defines the maximum allowable shift of the flow sensor output as being equal to the foregoing threshold of the dead zone and uses the flow sensor output as the zero point of the flow sensor output when the magnitude or absolute value of the flow sensor output is smaller than the foregoing threshold. It should be noted, however, that the flow sensor, operating on the principle of resistance measurement of a resistance strip exposed to the gas flow, produces an output signal of which value changes variously even when the gas flow rate is smaller than the foregoing threshold of the dead zone.
Further, the Japanese Laid-Open Patent Publication 8-271307 describes a composite flow meter that uses a fluidic sensor in addition to a gas flow sensor, wherein a discrimination is made periodically whether or not a calibration of the zero point should be conducted for the flow sensor output. When it is judged that it is the right time for calibration, the magnitude or absolute value of the flow sensor output is measured and a standard deviation of the absolute value thus measured is obtained. The zero point of the flow sensor output is thereby corrected based upon the standard deviation thus obtained.
According to the approach of the foregoing Japanese Laid-Open Patent Publication 3-264821, however, there is no guarantee that the flow rate suitable for the calibration of the flow sensor output occurs, as the value of the gas flow rate is ruled by the situation of the consumer. Thus, there is no guarantee that the desired calibration of the flow sensor output is achieved in the desired or scheduled time interval
Further, the inventor of the present invention has discovered, in an aging experiment in which the gas flow sensor is subjected to an artificial aging process, that the flow sensor output, which tends to increase with increasing gas flow rate, experiences a parallel translation at the beginning of the aging process as indicated in FIG. 1, wherein it can be seen in FIG. 1 that only the intercept of the output characteristic curve at the zero flow rate experiences a shift at the beginning of the aging process. There is no substantial change in the gradient of the output characteristic curve. It should be noted that the foregoing artificial aging experiment is conducted by increasing the temperature of the resistance heater in the flow sensor.
Thus, the initial flow sensor output y.sub.0, which is represented as EQU y.sub.0 =0.0628x+3.2524,
where x represents the flow rate, is changed to EQU y.sub.1 =0.0628x+2.9154
after the first artificial aging experiment. Thereby, it can be seen that the gradient of the flow sensor output characteristic curve does not change substantially and only the intercept at the zero flow rate (x=0) is changed.
On the other hand, it has been discovered, in a further aging experiment, which was conducted after the first aging experiment and induces a much larger aging effect in the resistance heater of the flow sensor, that the flow sensor output characteristic curve y.sub.2 after such a second aging experiment is represented as EQU y.sub.2 =0.0616x+5.5495.
In this case, it can be seen that there occurs a change in both the gradient and the intercept.
Summarizing above, it was discovered that the aging in the flow sensor output characteristic first occurs in the form of a displacement of the intercept or parallel shifting of the output characteristic curve. Thereafter, there occurs a change in both the intercept and gradient.
Thus, the approach of the Japanese Laid-Open Patent Publication 3-264821 raises a problem, in view of the prescribed tolerance of the flow rate measurement of .+-.3% for the flow rate range of 125-150 liter/hour, in that the tolerable error of the flow rate measurement, which takes a value of .+-.3.75 liter/hour for the flow rate of 125 liter/hour, well exceeds the width of the dead zone. It should be noted that the dead zone of the zero flow rate is set to 1.5 liter/hour or less. Thus, it is not possible to calibrate the deviation of the output characteristic of the gas flow sensor based upon the output of the fluidic sensor as taught by the foregoing Japanese Laid-open Patent Publication 3-264821.
Further, the process of the Japanese Laid-Open Patent Publication 3-264821 is ineffective for calibrating the time-dependent change of the flow sensor output in view of the necessity of calibrating both the gradient and the intercept of the flow sensor output characteristic when there is a severe aging. When the calibration is to be made on the gradient, which changes with time, the calibration for the intercept is no longer possible.
Further, the approach of the Japanese Laid-Open Patent Publication 4-208819 or of the Japanese Laid-Open Patent Publication 8-271307 raises a problem as noted below.
During an experimental investigation on the output characteristic of a gas flow sensor that detects the gas flow rate based upon a measurement of resistance of a heat-sensitive resistance strip, the inventor of the present invention has discovered that there exists a very large fluctuation in the flow sensor output when the actual gas flow rate is set to 0 liter/hour as indicated in FIG. 2. It is believed that this substantial fluctuation of the flow sensor output at the zero gas flow rate is caused by the accumulation of heat in the resistance strip of the flow sensor or by the convection of air occurring in the gas flow sensor.
It should be noted that the experiment of FIG. 2 is conducted by using a flow path having a width of 5 mm and a length of 25 mm, wherein the flow sensor is disposed on a top side of the flow path for measuring the flow rate of the gas or air through the flow path. It should be noted that the vertical axis of FIG. 2 represents the sensor output in terms of deviation from a preset flow rate, while the horizontal axis represents the actual flow rate in terms of liter/hour.
When the zero point calibration is made on the flow sensor thus configured according to the teaching of the Japanese Laid-Open Patent Publication 4-208818, there can occur a case in which the zero point is reset to the lowest point of the flow sensor output at the zero flow rate state. For example, the flow sensor output of -1.1 liter/hour may be defined as the new zero point of the flow sensor output. See FIG. 2. In such a case, the gas flow sensor would produce an output indicative of a non-zero flow rate even in the case in which the actual gas flow rate is zero. For example, there can be a case in which the gas flow sensor output assumes the maximum value of about 1.3 liter/hour as a result of the variation of the flow sensor output at the zero flow rate. In such a case, the flow sensor output as measured from the newly defined zero point takes a value of about 2.6 liter/hour, while this value substantially exceeds the upper limit of the dead zone of 1.5 liter/hour. Thereby, the flow meter that uses the gas flow sensor produces an output indicative of a non-zero gas flow rate in spite of the fact that there is no actual gas flow. A similar situation occurs also in the approach of the Japanese Laid-Open Patent Publication 8-271307.
Thus, conventional flow meters that use such a flow sensor have a problem in that they may produce an erroneous output indicative of the existence of a gas flow even in such a case in which there is no actual gas flow. This erroneous output can cause a serious problem when the flow meter is used for a gas utility meter, as the gas consumption fee is charged based upon the gas flow rate thus detected by the gas flow sensor.
Thus, it is concluded that, while it is important to conduct a zero point calibration in a gas flow sensor for use in a propane or other gas utility meter in view of the prescribed specification, it is not appropriate to use an arbitrary point in the dead zone for calibrating the zero point of the gas flow sensor output.